feat: add WebAuthn passkey authentication support

- Add passkey login as alternative authentication method
- Support multiple passkeys per user with custom names
- Backend: WebAuthn service, auth strategy pattern, API endpoints
- Frontend: passkey management UI in settings, login option
- Fix: convert downloader check from sync requests to async httpx
  to prevent blocking the event loop when downloader unavailable

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/src/module/api/__init__.py

@@ -4,6 +4,7 @@ from .auth import router as auth_router
 from .bangumi import router as bangumi_router
 from .config import router as config_router
 from .log import router as log_router
+from .passkey import router as passkey_router
 from .program import router as program_router
 from .rss import router as rss_router
 from .search import router as search_router
@@ -13,6 +14,7 @@ __all__ = "v1"
 # API 1.0
 v1 = APIRouter(prefix="/v1")
 v1.include_router(auth_router)
+v1.include_router(passkey_router)
 v1.include_router(log_router)
 v1.include_router(program_router)
 v1.include_router(bangumi_router)